Ultracold Quantum Gases Group

Welcome to the ultracold quantum gas research group at Aarhus University!

In our research we investigate the properties of atomic gases at extremely low temperatures. This allows us to understand the fundamental quantum mechanical behaviour of few- and many-particle systems.


Magnus with his price and the dean of Science and Technology at Aarhus University

Magnus wins the Deans Challenge

Magnus from the ultracold quantum gases group participated in this years annual case competition, Dean's Challenge. He had developed a rollator with automatic breaking and alarm system, which awarded him the first price in the HEALTH catagory. Congratulations to Magnus!

A news article in danish is available here.


Energy shift and decay rate of the polaron as a function of temperature, displaying an intriguing behavior around the critical temperature.

Finite-temperature behavior of the Bose polaron

After our recent observation of the Bose polaron, we are aiming to understand the quasiparticle in more depth.

Here we consider a mobile impurity immersed in a Bose gas at finite temperature. Using perturbation theory valid for weak coupling between the impurity and the bosons, we derive analytical results for the energy and damping of the impurity for low and high temperatures, as well as for temperatures close to the critical temperature Tc for Bose-Einstein condensation. These results show that the properties of the impurity vary strongly with temperature. The energy exhibits an intriguing non-monotonic behavior close to Tc, and the damping rises sharply close to Tc. We finally discuss how these effects can be detected experimentally.

Read our manuscript on the arXiv, and if you are interested, check out this other article on the topic as well.


The poster

Nils wins Poster Prize at CoQIPC 2017

The IST Austria recently hosted the Conference on Controllable Quantum Impurities in Physics and Chemistry (CoQIPC 2017). Nils went to the conference with his poster on the recent observation of bose polarons, and was awarded the Keysight Technologies Poster Prize of the poster competition. Congratulations!


Welcome Thomas

In this month, we welcome Thomas Guldager Skov, who starts as a Ph.D. in the MIX lab. He previously did a bachelors project with the MIX team, and is now ready to do full time research. Best of luck to Thomas!


Lars in his beloved lab

Goodbye Lars!

Lars Wacker found a job at Danish National Metrology Institute (DFM) in Copenhagen and will thus unfortunately leave our group. Lars did his PhD in the MIX lab and continued for a year and a half as a postdoctoral researcher. During his time here, he has been crucial in developing the MIX lab into its current great condition. Especially in the first production of KRb dual BECs, and in our few-body Efimov studies, he has made essential contributions. We wish him the best of luck in his future endeavours!


The portable setup of the rotating waveplate polarimeter

A portable rotating waveplate polarimeter

We describe the construction and performance of a polarimeter based on a quarter-wave plate rotated by a model airplane motor. The motor rotates at a high angular frequency of ω∼2π×160, which enables the polarimeter to monitor the polarization state of an incident beam of light in real-time. We show that a simple analysis of the polarimeter signal using the fast Fourier transform on a standard digital oscilloscope provides an excellent measure of the polarization state for many laboratory applications. The polarimeter is straightforward to construct, portable, and features a high-dynamic range, facilitating a wide range of optics laboratory tasks that require free-space or fiber-based polarization analysis. 

Read our manuscript in Review of Scientific Instruments.


Faraday image analysis

Sub-atom shot noise Faraday imaging of ultracold atom clouds

We have developed an imaging technique which can measure the atom number below the atom shot noise level. This work is closely related to our recent work on feedback stabilization of atom numbers. We use Faraday imaging which allows multiple images of the same cloud to be acquired. To describe the expected noise, we have developed a model based on photon shot noise and single atom loss. For clouds containing N~5×106 atoms, a precision more than a factor of two below the atom shot noise level is achieved.

Our manuscript on this work has been published in Journal of Physics B as part of a special issue on addressing quantum many-body problems with cold atoms and molecules.

Read the manuscript in Jour. Phys. B or on arXiv!


Three-body recombination in KRb mixtures displays a surprising lack of Efimov physics

Absence of observable Efimov resonances in ultracold KRb mixtures

In previous experiments with ultracold mixtures of potassium and rubidium, an unexpected non-universal behavior of Efimov resonances was observed. We have measured the scattering length dependent three-body recombination coefficient in ultracold heteronuclear mixtures of 39K-87Rb and 41K-87Rb and do not observe any signatures of Efimov resonances. This reestablishes universality of the three-body parameter across isotopic mixtures.

The article is published in Physical Review Letters and available on arXiv.


Coupled interferometer states in a Rb BEC

Improvement of an Atomic Clock using Squeezed Vacuum

Since the pioneering work of Ramsey, atom interferometers are employed for precision metrology. In a classical interferometer, atoms are prepared in one of the two input states, whereas the second one is left empty. In this case, the vacuum noise restricts the precision of the interferometer to the standard quantum limit (SQL). We have experimentally demonstrated a novel clock configuration that surpasses the SQL by squeezing the vacuum in the empty input state. As such, 0.75 atoms improve the clock signal of 10,000 atoms!

Published in Physical Review Letters and available on arXiv.



Goodbye Andrew!

Andrew got an industry job in New Zealand and is thus leaving the group. Throughout the last years, he has been a vital part of the Lattice lab, conducting remarkable research. Besides his work in the Lattice lab, his expertise has also been a major benefit for the rest of the Ultracold Quantum Gases Group. We wish him and his family the best in New Zealand!


Figure selected for Phys. Rev. A Kaleidoscope

A figure from our recent paper on phase separation and dynamics of two-component Bose-Einstein condensates was selected to be on display as part of the Phys. Rev. A Kaleidoscope!

Extra credit goes out to Kean Loon Lee who was main author on the paper.

Read the paper here or on arxiv




The Faraday imaging system and the FPGA at work

Choose the Number of Atoms in Your Cloud: Preparation of Ultracold Atom Clouds at the Shot Noise Level

Experiments with ultracold atoms inherently suffer from shot-to-shot atom number fluctuations which limit the precision. The UQGG Lattice team have demonstrated a technique for preparing a large cloud of a specific number of atoms with unprecedented low uncertainty. The usual atom number fluctuation of about 10% are reduced to below 0.1%!

During the experimental procedure, a series of non-destructive Faraday images probe the number of atoms in the cloud. A field programmable gate array provides online data analysis and performs feedback by removing atoms from the cloud until the desired number of atoms are reached. Finally, a second series of Faraday images confirm the number of atoms remaining in the cloud.

By creating similar atom clouds reproducibly, this newly developed technique can potientially improve the performance of atomic clocks and other high-precision measurements or simply just reduce the number of hours the typical graduate student has to spend in the lab to obtain data of sufficiently high quality.

The results have been published in Physical Review Letters as an Editors' Suggestion. Additionally, the work is featured in Physics, where a Focus article was written. The article can be found on arXiv as well.


Theis embracing a dear deer

Goodbye Miroslav! Welcome Magnus and Mikkel! Congratulations Theis!

Recently, Miroslav Gajdacz got a job at OFS Denmark, which develops optical fibres, and he thus left the group. He has done great research in the Lattice lab, where his main contribution was the implementation of Faraday imaging and the use of this to prepare ultracold atom clouds at the shot noise limit. Additionally, he has published several theory papers on atomtronics and the quantum speed limit. Based on his research, he obtained his PhD last fall and has since then continued his studies as a Posdoc. We wish him the best of luck in his future endeavours!

The next generation of PhD students in the UQGG has however arrived! Mikkel Berg Christensen has started in the Lattice lab. The recently developed precise production of ultracold samples will allow him to aid in exploring new frontiers of quantum gases. Additionally, Magnus Graf Skou has started in the MIX lab. The recent observation of polarons in a Bose-Einstein condensate has opened up for studies of quantum impurities in regimes never before realized.

Finally, Theis recently obtained his masters degree! He was a great asset for the group throughout the last year. His main contribution was made in the MIX lab where he aided to observe the Bose polaron. He will continue to work in the group as a research assistant.


The polaron in a Bose-Einstein condensate

Observation of Attractive and Repulsive Polarons in a Bose-Einstein Condensate

Mobile impurity particles interacting with a bosonic quantum environment play a central role in our understanding of nature and are fundamental for several important technologies such as organic electronics. It is therefore highly desirable to study impurity physics systematically and from a broad perspective as offered by cold atomic gases.

We present the experimental realization of long-lived impurity atoms in an atomic Bose-Einstein condensate. The energy of the impurity is measured and we find excellent agreement with theories that incorporate three-body correlations, both in the weak-coupling limits and across unitarity. For both strong repulsive and strong attractive interactions, our experimental results demonstrate the existence of a polaron quasiparticle.

The manuscript has been published in Physical Review Letters as an Editors' Suggestion, back-to-back with results from the group of Eric Cornell and Deborah Jin at JILA. Additionally, it can be found on arXiv.

Our results are also featured in several news outlets which appeal to a broader audience:

Physics Viewpoint: Bose Polarons that Strongly Interact

Quasipartikler som klumper i kold kvantesuppe (danish)



The Villum Foundation